Direction-dependent driving coupling between the rotor of a permanent-magnet synchronous motor and the working part

ABSTRACT

A direction-dependent driving coupling between the rotor of a permanent-magnet synchronous motor and a working part. The coupling comprises at least one first eccentric driving tooth and at least one cantilevered elastomeric element which are rigidly coupled to a first one of the two components between which motion is to be transmitted; the cantilevered element is arranged peripherally close to a cylindrical wall, of which it partially follows the shape, and the wall is formed by a housing, in which the rotor is arranged, and is motionless, at start-up, with respect to the motorized component. At least one second eccentric tooth is rigidly coupled to a second one of the two components between which motion is to be transmitted in such a position that the path between the parts causes it to interact with the first tooth in one direction and to interact with the cantilevered element in the opposite direction, producing a radial outward thrust which flexes the cantilevered element against the cylindrical wall, blocking it.

BACKGROUND OF THE INVENTION

The present invention relates to a direction-dependent driving couplingbetween the rotor of a permanent-magnet synchronous motor and theworking part.

It is known that permanent-magnet synchronous motors are bidirectional,i.e., at power-on the rotor can be induced equally to rotate clockwiseor counterclockwise.

This depends on the arrangement of the polarities of the rotor withrespect to the magnetic field that forms between the poles of the statorpack when the inductor winding is supplied with alternating current.

Although this is not a problem in the case of the actuation ofcentrifugal pumps with radial vanes, it becomes a considerablelimitation for centrifugal pumps and for fans having a particularconfiguration of the vanes and therefore a single direction of rotationof the impeller.

This is one of the main problems in the field of centrifugal machineswhich use permanent-magnet motors, and various electrical and mechanicaldevices have been proposed so far.

Among these, the motor pump unit described in EP 148343 is particularlysignificant.

This motor pump unit has a box-like body with a permanent-magnetsynchronous electric motor, the rotor whereof is arranged within asubstantially cup-shaped coaxial jacket arranged in the gap.

The rotor supports, at one end, an impeller whose hub lies coaxiallyinside the jacket.

An intermediate sleeve, driven by the rotor, is arranged between theimpeller on one side and the corresponding final section of the rotor onthe other side.

The intermediate sleeve comprises at least one cam which protrudesradially outwards; a tab co-operates with the cam on the internalsurface of the hub and protrudes radially at the path of the cam.

The tab is part of a flexible braking flap formed in the hub.

When the cam of the intermediate element encounters the tab of the hub,the flap flexes and rests against the jacket that contains the rotor,blocking the rotation.

The hub is also provided with at least one tooth which protrudesradially inwards and with which the cam makes contact by rotating in theopposite direction with respect to the above cited one, making it rotateand rotating the impeller rigidly with the rotor.

This motor pump requires precise calibration in the regions of contactbetween the cam and the tab that is part of the flexible flap in orderto avoid jamming.

The choice of the material that constitutes the flexible flap and thetab is in fact linked to the choice of the material of the impeller,with which it is monolithic since it is part of the hub.

The plastic material that constitutes the impeller, despite having acertain elasticity, must of course be predominantly rigid andaccordingly the necessary elasticity of the flexible flap is the resultof a compromise.

The fact of using mostly rigid plastic materials also has the inevitableconsequence of increasing noise caused by impacts between the componentsat start-up.

Another device for producing direction-dependent rotations, which inthis case is not linked to the problem of permanent-magnet electricmotors, is known from GB 361656, in which a driving shaft supports twodiametrically mutually opposite cams which are keyed to the shaft andwhich in one direction of rotation interfere with respective pawlspivoted in regions which are peripheral to the cams and supportrespective blocks arranged close to a cylindrical wall of a part to bedriven.

The cams have a circular external surface, while the blocks have aneccentric circular internal surface which is arranged close to thesurfaces of the cams.

Accordingly, rotary motion in one direction causes the cams to rigidlycouple to the pawls, whereas by rotating in the opposite direction theyproduce a rotation of the blocks with respect to the respective fulcrumsand lock them against the cylindrical wall.

In this case, although the flexible elements have been replaced withpivoted elements with respect to EP 148343, there is still an absoluteneed for perfect calibration of the geometry of the various parts, whichmust have a certain rigidity.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a direction-dependentdriving coupling between the rotor of a permanent-magnet synchronousmotor and the working part which eliminates the drawbacks noted above inconventional types.

Within the scope of this aim, a consequent primary object is to providea direction-dependent driving coupling which is unaffected by anyproblem in choosing the materials related to the rotor and/or theworking part.

Another important object is to provide a direction-dependent drivingcoupling which has an adequate response to the dynamics of start-up, sothat start-up in the chosen direction is immediate.

Another important object is to provide a direction-dependent drivingcoupling in which start-up impact noise is reduced.

Another object is to provide a direction-dependent driving couplingwhich, by using the same components and acting only on the assemblyprocess, allows to vary the direction of rotation at will.

Another object is to provide a direction-dependent driving couplingwhich has a particularly simple structure and assembly.

Another object is to provide a direction-dependent driving couplingwhich can be used both for working parts such as impellers ofcentrifugal pumps and for working parts such as the impeller of fans,i.e., with loads having considerably different inertias.

This aim, these objects and others which will become apparent hereafterare achieved by a direction-dependent driving coupling between the rotorof a permanent-magnet synchronous motor and a working part,characterized in that it comprises at least one first eccentric drivingtooth and at least one cantilevered elastomeric element which arerigidly coupled to a first one of the rotor and the working part betweenwhich motion is to be transmitted, said cantilevered element beingarranged peripherally close to a cylindrical wall, of which it partiallyfollows the shape, said wall being formed by a housing in which therotor is arranged and being motionless, at start-up, with respect to themotorized component, at least one second eccentric tooth being rigidlycoupled to a second one of the rotor and the working part between whichmotion is to be transmitted in such a position that the path between therotor and the working part causes it to interact with said first toothin one direction and to interact with said cantilevered element in theopposite direction, producing a radial outward thrust which flexes saidcantilevered element against said cylindrical wall, blocking allmovement.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will becomeapparent from the detailed description of two embodiments thereof,illustrated by way of nonlimitative example in the accompanyingdrawings, wherein:

FIG. 1 is a sectional perspective view of a permanent-magnet electricmotor, to the rotor of which a direction-dependent driving couplingaccording to the invention is coupled;

FIG. 2 is an exploded view of the assembly of the rotor and of thedriving coupling related to FIG. 1;

FIG. 3 is a transverse sectional view of the driving coupling;

FIG. 4 is an exploded view of the entire motor of FIG. 1;

FIG. 5 is a transverse sectional view of another embodiment of thedriving coupling;

FIG. 6 is a longitudinal sectional view of the embodiment of FIG. 5;

FIG. 7 is a longitudinal sectional view of a second permanent-magnetelectric motor provided with a second embodiment of the driving couplingaccording to the invention;

FIG. 8 is a sectional view of the rotor part alone, taken along theplane VIII—VIII of FIG. 7;

FIG. 9 is an exploded view of the components of the assembly of FIG. 8;

FIG. 10 is an exploded view of the assembly of the rotor and of a thirdembodiment of the driving coupling according to the invention;

FIG. 11 is a sectional view, taken along a longitudinal plane, of therotor and of the coupling of FIG. 10;

FIG. 12 is a sectional view, taken along the transverse plane XII—XII ofFIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 to 4 described above, said figures illustratea permanent-magnet electric motor, which is generally designated by thereference numeral 10 and is of a per se known type already described inItalian patent application No. PD97A000124 filed Jun. 12, 1997 in thename of the same Applicant.

In particular, the electric motor 10 comprises a stator 11, a laminationpack 12 with windings 13, and a rotor 14 which is arranged between twopoles 15 formed by the lamination pack 12.

The rotor 14 is constituted by an annular cylindrical permanent magnet16, on which a plastic element 17 is overmolded; the plastic elementforms an internal tang 17 a and end flanges 17 b.

The rotor 14 therefore is, as a whole, cylindrical with an axial hole 18in which a shaft 19 is inserted; the rotor 14 can rotate freely aboutthe shaft 19.

The shaft 19 is in turn connected to a supporting structure, generallydesignated by the reference numeral 20, which in this case is composedof three complementary elements, designated by the reference numerals21, 22 and 23 respectively, which are arranged coaxially and are joinedby means of screws which are not illustrated.

The supporting structure encloses the assembly constituted by the poles15, the rotor 14 and the shaft 19, allowing the shaft in any case toprotrude with an end 19 a to which a working part to be turned isrigidly coupled. The working part is shown in dashed lines anddesignated by the reference numeral 24, and can be constituted forexample by an impeller of a fan.

The rotor 14 is arranged inside a hermetic housing, generally designatedby the reference numeral 25, which is rigidly coupled to the shaft 19and is composed of a cup-shaped element 26 and of a hermetic plug 27.

The plug is arranged in a diametrically wider portion of the cup-shapedelement 26 and is provided with vanes 28 which act as a turbine for aliquid which is contained in the hermetic housing 25 in which the rotor14 is arranged.

A bladed impeller 29, which in this case acts as a pump, faces the vanes28 of the plug 27 in the wider part of the hermetic housing 25 and iscoupled to the rotor 14 by means of a direction-dependent drivingcoupling according to the invention, which is generally designated bythe reference numeral 30 in the figures.

The coupling 30 comprises at least one first driving tooth 31 and aninternally hollow tang 32; both are monolithic with the correspondingflange 17 b of the rotor 14 from which they protrude axially, and thetooth is eccentric (i.e., it is radially shifted with respect to theshaft 19 of the rotor 14).

An annular element 33 made of an elastomer such as rubber is arranged soas to surround the tang 32 and one of its peripheral portions isadjacent to a radially internal portion of the first tooth 31.

Two radial tabs, designated by the reference numerals 34 and 35,respectively protrude from the annular element 33 and are arranged atthe ends of the first tooth 31.

At least one cantilevered element 36 protrudes from one of the radialtabs 34,35, in this case in particular the tab designated by thereference numeral 34, and is peripherally close to the cylindrical wallof the cup-shaped element 26, of which it partially follows the shape.

In particular, the profile of the cantilevered element 36 has a circularportion 37, which is indeed adjacent to the wall of the cup-shapedelement 26, and a straight portion 38, which gives it a wedge-likeshape.

The cantilevered element 36 forms, together with the annular element 33with which it is monolithic, a hollow region 39 in which at least onesecond tooth 40 can partially enter. The second tooth 40 protrudeseccentrically with respect to the shaft 19 from the bladed impeller 29.

The relative path, in one direction of rotation, between the secondtooth 40 and the cantilevered element 36 causes the cantilevered elementto radially push it outwards, flexing it against the cylindrical wall ofthe cup-shaped element 26 and blocking its motion.

For rotations in the opposite direction, which are caused at start-up bythe initial orientation of the magnetic field or by its reversal afteran attempt in the opposite direction combined with the rebound effectproduced by the elastomeric material, the second tooth 40 rests againstthe radial tab 35 and therefore interacts with the first tooth 31,making it rotate.

Accordingly, blocking occurs for attempted rotations in one direction,whereas freedom is allowed in the opposite direction.

Over a certain portion of a complete rotation, approximately 180xdegrees, the rotor 14 is disengaged from the load constituted by thebladed impeller 29 and can therefore start freely before rotationallydriving the impeller.

The impeller 29, together with the vanes 28, forms a viscous coupling inwhich the impeller provides kinetic energy to the working fluidcontained in the housing 25 only in the direction in which the rotor 14rotates.

The kinetic energy is converted into pressure energy (head) by the shapeof the vanes of the meridian ducts of the impeller 29.

The working fluid moved in the vane ducts of the impeller 29 starts tocirculate in the ducts of the vanes 28 of the plug 27, which accordinglystart to rotate.

Thus, the working part 24 is made to rotate.

The viscous coupling, combined with the driving coupling 30, ensures thedirection-dependent start-up of the motor in conditions which are fullysimilar to those of an asynchronous motor.

At this point it should be stressed, with reference to what has beenprovided so far by the state of the art, that the element that providesdirection-dependent motion (the annular element 33 with radial tabs 34and 35 and a cantilevered element 36) is an independent part which isnot integrated with the other parts of the coupling.

A first effect of this is a simplification of the production mold, butin particular this allows to provide the element using the most adaptedmaterial in terms of mechanical characteristics and of impact cushioningin order to obtain low noise levels.

This independence makes the assembly less dependent on a correctcalibration of the geometries, since it is simply necessary to obtain,upon contact between the parts, a component of radial force which causesthe cantilevered element 36 to flex outwards.

The provision of the element that determines direction-dependent motionby using an elastomer such as rubber allows to utilize the high frictioncoefficient of the rubber to ensure initial quick and quiet blocking orallows to determine at will the degree of deformation of thecantilevered element 36 in a simple way by acting on the hardness of thematerial or on the geometry of the parts with no risk of jamming of thesystem, achieving adequate start-up responses.

It should also be noted that the radial tab 35, in the case of a correctdirection of rotation at start-up, acts as a shock-absorber andtherefore as a noise damping element.

It is furthermore particularly important that the annular element 33 canbe fitted equally with the radial tabs 34 and 35 on either side of thefirst tooth 31; this allows to decide the direction of rotation simplyby changing the orientation with which it is assembled.

The configuration of the load (for example the vane curvature of animpeller) must of course be adequate.

It should also be noted that the coupling has no articulated parts whichmay be easily subjected to jamming caused by the deposit of dirt.

With reference now to the above FIGS. 5 and 6, in a different embodimentof the coupling the first tooth, now designated by the reference numeral31 a, covers an angle which is smaller than the angular space betweenthe radial tabs, now designated by the reference numerals 34 a and 35 a,of the annular element, which is now designated by the reference numeral33 a.

The annular element is provided, between the tabs 34 a and 35 a, with atang 37 a which has a semi-circular cross-section and is inserted in acomplementarily shaped hollow 38 a of the first tooth 31 a, with whichit makes contact.

At start-up, therefore, before the first tooth 31 a makes contact withthe tab 35 a and before the consequent driving, there is a frictioneffect between the tang 37 a and the hollow 38 a in relative motion,consequently damping the contact impact.

Friction can of course also be provided by simply acting on theinterference between the annular element 33 a and the tang arrangedinside it, which is now designated by the reference numeral 32 a.

With reference now to FIGS. 7 to 9 described above, a permanent-magnetelectric motor of the previously described type which is connected to animpeller of a centrifugal pump is illustrated.

In particular, a permanent-magnet rotor 110 is arranged inside atube-shaped chamber 111 (housing) which is arranged in the gap betweentwo poles formed with a lamination pack 111a which are connected towindings (not shown), and can rotate freely with respect to a shaft 112which is fixed axially to the chamber.

The rotor 110 is composed of an annular permanent magnet 113 and of anovermolded element 114 made of plastics, which forms an internal tang114 a and end flanges 114 b.

A direction-dependent driving coupling according to the invention,generally designated by the reference numeral 116 in a secondembodiment, is arranged between the rotor 110 and the bladed impeller115 which constitutes the working part.

In particular, the impeller 115, which has a hub 117 fitted on anextension 118 of the internal tang 1141a of the rotor 110, with respectto which it is in any case able to rotate freely, supports an annularelement 119 which is rigidly coupled to the hub 117 and is of the sametype as the preceding element 33. A first tooth 120 protrudes radiallyfrom the annular element 119 and in turn is rigidly (monolithically)provided with a cantilevered element 121, i.e., with an element which isperipherally close to the cylindrical wall of the tube-shaped chamber111, of which it partially follows the shape and which has asubstantially wedge-shaped free end.

A hollow region 122 is formed between the cantilevered element 121 andthe annular elastomeric element 119.

As regards again the annular element 119, it is provided with grooves123 which are complementary to grooves 124 of the hub 117 in order torigidly rotationally couple it to the hub.

The annular element 119 also is provided with axial hollows 125 whichconstitute seats for positioning raised portions 126 of the hub 117 inorder to correctly position it and fix it axially.

The driving coupling 116 also comprises a second tooth 127 whichprotrudes from a corresponding flange 114 b of the overmolded element114 of the rotor 110 and is arranged so that its path, in one direction,causes it to make contact with the first tooth 120, rotating it. In theopposite direction, the path of the second tooth causes it to makecontact with the cantilevered element 121, partially entering the hollowregion 122, causing a radial outward thrust which flexes it against thecylindrical wall of the chamber 111, blocking it.

It should be noted in this case that the cantilevered element 121 isrigidly coupled to the working part, differently from the precedingcase, in which it was rigidly coupled to the rotor.

In practice, this second embodiment also has shown the same advantagesmentioned earlier for the first embodiment, except for the fact that inthis case it is not possible to change, during assembly, the directionof rotation owing to the particular configuration of the coupling.

With reference to FIGS. 10, 11 and 12, a rotor 210 is composed of apermanent magnet 211 which has a cylindrical annular structure and onwhich a plastic element 212 is overmolded which forms an internal tang212 a and, at the ends, flanges 213.

Accordingly, as a whole the rotor 210 has a cylindrical structure withan axial hole 214 in which a shaft 215 is inserted; the rotor 210 canrotate freely with respect to the shaft 215.

The shaft 215 has a free end 215 a which protrudes from the rotor 210and with which a working part is rigidly associated, as will becomeapparent hereinafter.

A direction-dependent driving coupling, in this case in a thirdembodiment designated by the reference numeral 216 in the above figures,is interposed between the rotor 210 and the working part.

The coupling 216 comprises a first driving tooth 217 which protrudesmonolithically with respect to the flange 213 of the rotor 210 parallelto the shaft 215 in a radial eccentric position, i.e., shifted withrespect to said shaft 215.

A curved flap 218 protrudes monolithically from the flange 213 from aposition which is rotated substantially through a right angle withrespect to the first tooth 217; the flap is parallel to the shaft 215and acts as an extension of the external structure of the rotor 210.

A body 219 is overmolded on the shaft 215 proximate to the free end 215a, has a cylindrical structure and is axially crossed by the shaft 215.

The body 219 has, on the side directed toward the rotor 210, an annularextension 220 which lies axially and has a smaller diameter and is alsocrossed with a rigid coupling by a corresponding portion of the shaft215.

A second tooth 221 protrudes from the body 219, is parallel to theannular extension 220 and is arranged eccentrically in a radialposition, i.e., shifted with respect to the shaft 215 of the rotor 220.

In particular, the external path covered by the second tooth 221 due tothe rotation of the shaft 215 and of the body 219 that is rigidlycoupled thereto does not interfere with the curved flap 218 which isrigidly coupled to the rotor 210.

The driving coupling 216 comprises an annular element 222 made of anelastomer such as rubber which surrounds, on assembly, the extension 220and has in particular a first radial tab 223 and a second radial tab 224which are arranged at the ends of the second tooth 221.

A cantilevered element 232, in particular, protrudes from the secondradial tab 224 and is peripherally close to the cylindrical wall,schematically designated by the reference numeral 233, of a housing ofthe rotor 210.

In this manner, while the bulk occupied externally by the first radialtab 223 does not interfere with the curved flap 218, which issubstantially the extension of the external structure of the rotor 210and does not interfere with the cylindrical wall of the housing,schematically shown by the dashed line 233, the overall shape of thesecond tab 224, produced by said cantilevered element 232, widensstarting from the region proximate to the second tooth 221 until itinterferes with the wall 233 of the housing.

The free end 215 a of the shaft 215 which is rigidly coupled to theworking part, constituted for example by the bladed impeller of a fannot shown in the figures for the sake of simplicity, is supported by acup-shaped element 226 on the bottom of which there is a hole which iscrossed by the free end 215 a.

A thrust bearing 227, which absorbs the axial vibrations, and a slidingbearing, designated by the reference numeral 228 and of a per se knowntype, are arranged inside the cup-shaped supporting element 226.

When the rotor 210 starts, the first driving tooth 217 and the curvedflap 218 are turned, together with the flange 213 which is rigidlycoupled to the rotor 210, in a direction which depends on the initialorientation of the magnetic field.

If the direction of rotation is such as to bring the first driving tooth217 to rest against the first radial tab 223, it also directly actuatesthe second tooth 221, which is in fact locked between the first radialtab 223 and the second radial tab 224, consequently rotating the body219 as well and accordingly rotating the shaft 215 on which the workingpart is keyed.

The curved flap 218 in fact is wedged, just before contact between thefirst tooth 217 and the first tab 223, between the second radial tab 224and the cylindrical wall 223 of the housing, moving the cantileveredelement 232 away from it so as to prevent contacts and consequentfriction.

Vice versa, if the direction of rotation of the first tooth 217 is suchas to make it rest against the second radial tab 224, the tab isconsequently subjected to an outward thrust which widens thecantilevered element 232 against the cylindrical wall 233 of thehousing, producing considerable friction between the moving parts owingto the high friction coefficient of the annular element 222, which ismade of elastomer.

It is interesting to note that by virtue of the structure of thecoupling 216, if the direction of rotation is not the intended one, theresult is a quick and most of all silent speed reduction and blockingaction.

It is also interesting to note that when the first radial tab 223,pushed by the first tooth 217, rotates the second tooth 221 concordantlywith it, it performs a shock-absorbing function, cushioning any impactand consequent noise.

In practice it has been observed that the intended aim and objects ofthe present invention have been achieved.

The invention thus conceived is susceptible of numerous modificationsand variations, all of which are within the scope of the inventiveconcept.

All the details may furthermore be replaced with other technicallyequivalent elements.

In practice, the materials used, so long as they are compatible with thecontingent use, as well as the dimensions, may be any according to therequirements.

The disclosures in Italian Patent Application No. PD98A000003 from whichthis application claims priority are incorporated herein by reference.

What is claimed is:
 1. A direction-dependent driving coupling betweenthe rotor of a permanent-magnet synchronous motor and a working part,comprising at least one first eccentric driving tooth and at least onecantilevered elastomeric element which are rigidly coupled to a firstone of the rotor and the working part between which motion is to betransmitted, said cantilevered element being arranged peripherally closeto a cylindrical wall, of which it partially follows the shape, saidwall being formed by a housing in which the rotor is arranged and beingmotionless, at start-up, with respect to the motorized component, atleast one second eccentric tooth being rigidly coupled to a second oneof the rotor and the working part between which motion is to betransmitted in such a position that the path between the rotor and theworking part causes it to interact with said first tooth in onedirection and to interact with said cantilevered element in the oppositedirection, producing a radial outward thrust which flexes saidcantilevered element against said cylindrical wall, blocking it.
 2. Thecoupling according to claim 1, wherein said cantilevered elementprotrudes from an annular element which is rigidly coupled to said firsttooth.
 3. The coupling according to claim 2, wherein said annularelement is arranged radially internally with respect to said first toothand has two tabs arranged at its ends, said cantilevered elementprotruding monolithically from one of said tabs.
 4. The couplingaccording to claim 1, wherein said cantilevered element is radiallyinterposed, with one of its portions, between said second tooth and saidwall.
 5. The coupling according to claim 1, wherein said first tooth isrigidly coupled to the rotor of the motor and wherein said cantileveredelement is rigidly coupled to said first tooth, said cylindrical wallbeing rigidly coupled to the working part.
 6. The coupling according toclaim 5, wherein said second tooth is monolithically coupled to a bladedimpeller which acts as a pump and faces a bladed impeller which acts asa turbine and is rigidly coupled to said wall since it is part, togetherwith said wall, of a rotatable hermetic housing in which said rotor isarranged.
 7. The coupling according to claim 1, wherein saidcantilevered element is monolithic with said first tooth.
 8. Thecoupling according to claim 7, wherein said first tooth is rigidlycoupled to said working part and said second tooth is rigidly coupled tosaid rotor.
 9. The coupling according to claim 8, wherein said workingpart has a hub which rigidly supports an annular elastomeric elementfrom which said first tooth protrudes radially, said annular elementsupporting said cantilevered element which is rigidly coupled theretoand lies peripherally close to the cylindrical wall of a tube-shapedchamber in which said rotor is placed and which is part of thesupporting structure of said rotor.
 10. The coupling according to claim9, wherein said second tooth is monolithic with said rotor.
 11. Thecoupling according to claim 9, wherein said annular element isinternally provided with grooves which are shaped complementarily togrooves of said hub which are adapted to rigidly couple it to said hubin rotation, said annular element being also provided with axial hollowswhich constitutes seats for positioning raised portions of said hub forcorrectly positioning and axially fixing it.
 12. The coupling accordingto claim 1, wherein at least its free end is substantially wedge-shaped.13. The coupling according to claim 1, wherein between said annularelastomeric element and the one of said two components that can be inrelative motion therewith at start-up, on the side that corresponds tothe correct direction of rotation there are parts which are in slidingcontact over a preset angle so as to provide a friction effect in orderto cushion the start-up impact.
 14. The coupling according to claim 13,wherein the angle covered by said first tooth is smaller than theangular space between said tabs that protrude from said annular element,so as to provide a friction-controlled relative motion at start-up. 15.The coupling according to claim 1, wherein said first tooth is rigidlycoupled to the rotor of the motor and said second tooth is rigidlycoupled to said working part, a curved flap protruding from said rotorfrom a position which is rotated with respect to said first tooth, towhich it is rigidly coupled, said flap extending the external structureof the rotor and wedging itself, when said first tooth actuates saidsecond tooth rigidly coupled to the working part, between saidcantilevered element and said cylindrical wall so as to prevent allcontact, said cylindrical wall being fixed with respect to saidmotorized component.
 16. The coupling according to claim 6, wherein saidhousing in which the rotor is arranged is fixed with respect to thestator.